The forming of titanium alloys into complex configurations by present day processes, for forming parts requiring large tensile elongations, is extremely difficult and in some cases cannot be achieved. Limited tensile elongations, high yield, and moderate modulus of elasticity impose practical limits for ambient temperature forming, and excessive spring-back frequency requires elevated temperature creep sizing. In some parts, forming is done in a 1200.degree.-1400.degree.F temperature range to increase the allowable deformation and to minimize spring-back and sizing problems. However, even with the use of these moderately high temperatures, an extremely expensive integrally heated double-action forming tool is required. Even with these advanced techniques, forming of titanium alloys is still severly limited and compromises in the design of structural hardware are often necessary with attendant decrease in efficiency and increase in weight.
For several years it has been known that titanium and many of its alloys exhibit superplasticity. Superplasticity is the capability of a material to develop unusually high tensile elongations with reduced tendency toward necking, a capability exhibited by only a few metals and alloys and within a limited temperature and strain rate range. Titanium and titanium alloys have been observed to exhibit superplastic characteristics equal to or greater than those of any other metals. With suitable titanium alloys, overall increase in surface area of up to 300 percent are possible.
The advantages of superplastic forming are numerous: Very complex shapes and deep drawn parts can be readily formed; low deformation stresses are required to form the metal at the superplastic temperature range, thereby permitting forming of parts under low pressures (as 15 psi) which minimize tool deformation and wear, allows the use of inexpensive tooling materials, and eliminates creep in the tool; single male or female tools can be used; no spring-back occurs; no Bauschinger effect develops; multiple parts of different geometry can be made during a single operation; very small radii can be formed; and no problems with compression buckles or galling are encountered. However, prior to applicants' invention superplastic forming of titanium and similar reactive metals was impractical because of the high forming temperatures required and the relatively long time in forming. Titanium at the superplastic forming temperature has a strong affinity for most elements. The heating and forming atmosphere is critical to titanium cleanliness which is particularly sensitive to oxygen, nitrogen, and water vapor content. Unless the titanium is protected, it becomes embrittled and its integrity destroyed. Coating materials cannot be used for protection at the superplastic forming temperatures as the coatings and associated binders react with and contaminate the titanium alloy in any type of environment.
The present invention relates generally to a method and apparatus for superplastic forming of metals in a controlled environment. More specifically, the present invention relates to superplastic forming of metal blank into a desired shape by heating the metal blank in a controlled environment and applying a fluid pressure loading to the metal blank causing it to form against a shaping member.
A method for superplastic forming of metals has been disclosed in U.S. Pat. No. 3,340,101 to Fields, Jr., et al. This patent discloses heating or otherwise conditioning a metal to exhibit its effective strain rate sensitivity and then placing the metal in an apparatus for forming. Forming is usually accomplished by a vacuum exerting tensile stress on the metal. However, a male die member can be utilized to initially deform the metal before application of the vacuum, or the male die member can be used in combination with the application of positive pressure. However, this method would not be suitable for superplastic forming of titanium because of the contamination that would result to the surface integrity of the metal due to the heating and forming without a controlled environment. In fact, the patent does not list titanium as one of the metals having superplastic properties and discusses forming temperatures in the range of 600.degree. F. as opposed to the approximately 1450.degree.-1850.degree. F. required by titanium and its alloys. No mention is made in the patent as to protection from contamination. Additionally, forming time, especially with thicker metal sheet is quite lengthy as the amount of differential pressure is limited.
U.S. Pat. No. 3,605,477 to Carlson discloses apparatus for hot forming titanium alloy blanks where the blanks are coated with a high temperature lubricant, preheated to a forming temperature of about 1,000.degree. to 1,500.degree. F., removed and placed in forming equipment in contact only with mated heated forming tools. The forming equipment is maintained at the forming temperature during forming. It is disclosed to use an argon atmosphere in the heater when preheating the titanium blanks to prevent contamination. However the blank is removed from the heater to separate forming equipment where it is formed into the desired shape without the benefit of the controlled environment. For protection, a high temperature lubricant is formed on the titanium sheet. This method while suitable for hot forming of titanium, would be impractical and unsuccessful for superplastic forming. In the superplastic forming temperature range of approximately 1450.degree. to 1850.degree. F, the high temperature forming lubricant itself contaminates the titanium sheet regardless of the environment. In any case, the heater is separate from the forming apparatus and once the titanium sheet is removed from the heater it would be contaminated.